If so, that should be instant, so there shouldn’t be any loading necessary. If you can post a Sandcastle (https://cesiumjs.org/Cesium/Build/Apps/Sandcastle) example of the delay you’re seeing that’d help.
Unfortunately, this isn’t something that CesiumJS supports. The ready promise is intentionally hidden in the Entity API. You can see the discussion on that here:
You could use the primitive API as described in that issue, but then you won’t be able to use trackedEntity.
If the model takes a significant time to load, this might be a good reason to tile it up into 3D Tiles (which you can do with Cesium ion https://cesium.com/ion/)